This invention relates to motor control apparatus for controlling motor rotation and, more particularly, to a motor control apparatus for maintaining a constant rotational speed of a DC servo-motor.
A motor, with which a constant rotational speed being unsusceptible to changes of the load and other external disturbances can be steadily obtained, is desired for driving the capstan of a tape recorder or the like. While various types of motors have hitherto been developed to meet this requirement, DC servo-motors which can be operated with low voltage and with which the rate of speed change with changing load is low, are suited for use with small-size tape recorders of battery powered type such as microcassette taper recorders. Japanese Patent Publication No. 32126/79 discloses a motor control circuit for such a DC servo-motor.
FIG. 1 is a block diagram which is a combination of FIGS. 4 and 6 in the afore-mentioned publication. Here, by assuming EQU K.sub.2 =1 (1)
and ##EQU1## equation (15) shown in the said publication, i.e., the transfer function H.sub.1 (S) of the circuit from point A to point H in FIG. 1, can be expressed as ##EQU2## In equation (3), the constant factor independent of the complex number parameter S, i.e., the gain G1, is ##EQU3## From equation (2), equation (4) can be modified as ##EQU4## Equation (5) indicates that in the prior-art motor control circuit shown in FIG. 1 the gain G1 depends upon the capacitances C3 and C4.
In the actual circuitry, capacitors of the order of .mu.F are often used for the capacitors C3 and C4. This provides difficulty in integrating the circuitry of FIG. 1 into a bipolar IC. For this reason, the capacitors C3 and C4 have to be provided as external component parts of the IC. These capacitors C3 and C4 having the capacitances of the order of .mu.F may be fabricated as film capacitors or chemical capacitors. The former capacitors are large in size and expensive. The latter capacitors are far superior to the former capacitors insofar as their size and cost are concerned. However, the capacitance fluctuation or variation of chemical capacitors is considerable, and therefore their use is prone to provide considerable fluctuation of the aformentioned gain G1. In other words, the use of chemical capacitors causes the number of adjustment steps in the mass-production of a tape recorder to increase and also causes the quality or performance of the product to deteriorate (particularly characteristics of the speed accuracy and the wow/flutter are liable to deteriorate). From the above grounds, it is a conclusion that a motor control apparatus which is small in size and inexpensive and features little fluctuations of characteristics can hardly be obtained through the IC fabrication of the circuitry shown in FIG. 1.
The motor control circuit disclosed in the Japanese Patent Publication No. 32126/79 mentioned above has a further drawback. This lies in the lower limit of the working source voltage range. FIG. 2 corresponds to FIG. 8 in the publication. In this circuitry, supposing that transistors Q4 and Q7 and a constant current source IS are constituted by ordinary silicon bipolar transistors. The base-emitter threshold voltage V.sub.EB of a silicon transistor at room temperature is about 0.6 V. Also, supposing that the constant current source IS has the same circuit configuration as a transistor Q10 as shown, the supply voltage V.sub.IS for the source IS will require about 0.5 V or more. This is because, if the voltage V.sub.IS becomes lower than 0.5 V, the collector output impedance of the transistor constituting the source IS is greatly reduced so that the constant current characteristic can no longer be obtained. This is obvious from the collector current versus collector voltage characteristics of the conventional silicon transistor taking the base current as a parameter. From the above grounds, where V.sub.EB =0.6 V and V.sub.IS =0.5 V, the supply voltage Vcc to a controlled constant current source 109 has to be at least 1.7 V. It seems, in conclusion, that for ensuring the steady and stable circuit operation the power supply voltage supplied to the motor control circuit shown in FIGS. 1 and 2 at room temperature has to be about 2 V or more.